1. Technical Field
The present invention relates to an automobile battery and a method of manufacturing plates thereof.
2. Description of the Related Art
Batteries include secondary batteries or storage batteries (hereinafter, referred to as “batteries”) which are reusable because a series of procedures of supplying the charged current via discharge, wherein voltage drop occurs during discharge, performing discharge until the inner voltage of the battery reaches the set lowest voltage, performing charge before the permissible lowest voltage, and re-supplying the current via discharge are repeated an allowed number of times.
Batteries are classified into a variety of kinds, depending on the materials used for the anode, cathode, and electrolyte (which indicates an electrolytic solution), and a battery wherein lead (Pb) is used for electrodes and sulfuric acid is used as an electrolyte is called a lead battery.
A lead battery having the present configuration was developed by Plant, in France in 1860, and is advantageous because the voltage is the highest among battery systems using a liquid electrolyte, a variety of current values are supplied in the relatively wide temperature range, and energy efficiency is high to the level of 80% or more. Furthermore, a lead battery has a longer lifespan, offers better storage performance, and has a lower price, and is recyclable and thus eco-friendly, compared to the other kinds of batteries. Hence, this battery has been steadily used for about 150 years, from the time it was developed to the present day.
Electrodes of the lead battery include plates and poles for anode and cathode, and respective plates are physically and electrically separated by separators, and are received or placed in a battery case along with the electrolyte.
The anode and cathode plates of the lead battery are composed mainly of lead and should have a predetermined thickness, and thus are regarded as considerably heavy elements.
Examples of such a lead battery include a vehicle battery made so as to be adapted for vehicles to handle conditions of extreme impact, vibration and tilt, and an industrial battery made so as to output a large current in a fixed place.
A lead battery operates based on a principle in which charge and discharge are repeated via a reversible reaction of, for example, PbO2+H2SO4PbSO4+2H2O, and the produced electricity is discharged and output and the input electricity is charged and stored.
A lead battery utilizes a chemical action in which lead dioxide (PbO2) used for the plates and sulfuric acid (H2SO4) used for the electrolyte are converted into lead sulfate (PbSO4) and water (H2O). Upon conversion into lead sulfate (PbSO4) and water (H2O), electricity is produced and discharged, and upon charge, the opposite substitution takes place, and these procedures are repeated.
Whenever such a chemical reaction progresses, heat is generated from the inside of the battery, whereby part of the electrolyte may evaporate and simultaneously a small amount of hydrogen gas is generated. To prevent explosion due to the gas, the produced gas should be exhausted to the outside of the battery.
Briefly, a lead battery generates hydrogen gas and heat during charge and discharge, and also, an electrolyte may evaporate by heat to generate gas, which is then exhausted so that explosion of the battery due to the gas is prevented.
The lead battery having a closed case includes a gas outlet, but the amount of the electrolyte is gradually decreased via exhaust of the gas and thus the electrolyte should be periodically supplemented.
Also, a vehicle battery is exposed to vibration and tilt generated during driving. As such, the electrolyte should be prevented from leaking via the gas outlet.
Compared to the other kinds of batteries, such a lead battery is problematic because of a lower energy density, which has become an obstacle to widening the range of application of such a battery or the end uses thereof
For instance, lead batteries for electric vehicles (EVs), the future demand of which is expected to drastically increase, are capable of a performance of about 20 to 30 Wh/kg, which is known to be an energy density suitable for driving a distance of about 80 to 100 Km.
Electric vehicles using such conventional lead batteries have to be equipped with a large number of heavy lead batteries to increase the driving distance, and the fuel efficiency of vehicles may decrease due to the weight of lead batteries. Hence, much attention is paid to the development of light lead batteries having high energy density.
Conventional lead batteries have been mass produced because plates therefor are formed using a casting process.
Conventionally, plates are manufactured in such a manner that a lead solution is gravity fed into a cast mold having a negative pattern corresponding to a shape of a plate, cooled and removed from the mold. The metal structure of the plates formed using a gravity casting process is magnifiedly observed to the extent that its grain boundaries may appear.
The plates, which are manufactured using the conventional casting process, make it difficult to maintain the thickness thereof uniform, and thus grain boundaries of the metal structure are shown to be comparatively large.
However, as charge and discharge of the battery are repeated, the plates may corrode along the grain boundaries of the metal structure thereof When corrosion takes place at the grain boundaries having a large structure, the grain boundaries having a large structure are separated from the plates, making it difficult to maintain the shape of the plates, undesirably shortening the lifespan of the battery.
To partially solve problems of separation of the grain boundaries having a large structure due to corrosion, a precious metal such as silver (Ag), barium (Ba), etc., may be added to lead, thereby forming a small metal structure between the grain boundaries having a large structure, consequently increasing the binding force between the metal structures.
This improved technique is advantageous because corrosion resistance of the lead plates may be increased, thus prolonging the lifespan of the battery.
However, the manufacturing cost of the plates may be increased due to the use of precious metal, undesirably increasing the price of the lead battery.
Also, problems of the weight of the lead battery not being reduced due to the weight of the plates manufactured using the conventional casting process remain unsolved.
To decrease the weight of the lead battery, techniques for manufacturing plates having a grid or mesh shape, instead of heavy plates manufactured using the conventional casting process, have been developed.
FIG. 1 illustrates a plate cast by gravity feeding a lead solution into a cast mold according to a conventional technique, and FIG. 2 illustrates a plate formed using a gravity casting process according to another conventional technique.
As illustrated in FIG. 1, a lead solution is gravity fed into a cast mold having a negative pattern corresponding to a plate having a grid (mesh) shape, cooled for a predetermined period of time, and then removed from the mold.
In the case of using such a casting process, two plates may be simultaneously formed using a single cast mold to increase productivity.
The two cast mesh-shaped plates are cut and separated, and unnecessary portions are trimmed, thus completing individual plates.
FIG. 2 illustrates the mesh-shaped plate separated after a gravity casting process, wherein the grid shape is different
However, such conventional techniques are problematic because a series of processes including gravity feeding the lead solution into a cast mold, and cooling and extracting it require a period of time of, for example, about 4 to 5 sec or longer, undesirably increasing the production cycle time.
Moreover, the gravity casting process does not make the structure of the plate dense, resulting in poor corrosion resistance, and ultimately making it difficult to solve problems in which the lifespan of the plate is short.
Korean Patent No. 10-0289221 issued on Feb. 16, 2001, wherein the above problems are partially solved, discloses a battery grid, a plate, and a lead-acid battery made using the grid and the plate.
In this improved conventional technique, the lead plate is cast, processed to have a plurality of cuts having a uniform size thereon, and expanded at a rate of about 100 to 150 feet per minute, thus reducing the weight of the plate.
However, this conventional technique is also problematic because the metal structure is not dense due to formation of the lead plate using a gravity casting process, undesirably causing poor corrosion resistance, a short lifespan of the battery, a complicated production process, and a long production time, resulting in increased manufacturing costs.
In addition, Korean Patent Application No. 10-2009-7019301 filed Sep. 15, 2009 discloses a cathode grid for a battery.
In such a conventional technique, the cast lead plate is punched, so that the size of the plate is made uniform and high productivity may be obtained, but the consumption of lead may increase due to the punched portions, undesirably increasing the manufacturing cost. Furthermore, because the lead plate is formed using a gravity casting process, the metal structure is not dense and thus poor corrosion resistance may be attained, and the expectable lifespan of the battery may decrease.
Therefore, there are needs to develop a lead battery having high applicability, making it very useful as the energy source for electric vehicles, wherein the total weight of the lead battery may be reduced, and corrosion resistance of the plates may increase, thus prolonging the lifespan of the battery, decreasing the manufacturing cost, and increasing the energy density.
Moreover, there are needs to develop techniques for preventing a decrease in the amount of the electrolyte due to gas exhaust, and preventing the corrosion of the plates due to external air.